With the developments in technology, portable electronic products are more and more popular, especially those with a camera function. A photosensitive unit of a common optical system includes a charge-coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS). With the improvements in the technology of a semiconductor manufacture process, the photosensitive unit is of an increasing smaller pixel size. Correspondingly, it requires a smaller image lens adapted for the photosensitive unit and guaranteeing high imaging quality at the same time. In the past, the imaging quality is commonly improved by adding lenses, which is, however, adversely affects miniaturization of the imaging lens. In addition, the field angle is narrowed for improving the imaging quality, which is also adversely affects a wide-angle feature of the imaging lens.